Biological wastewater purification with a downstream precipitation stage

ABSTRACT

Wastewater containing impurities refractory to secondary treatment is first introducted into an aerobic biological treatment stage operating with an oxygen-enriched stream and subsequently into a precipitation stage where the biologically treated wastewater is combined with Ca(OH) 2  to precipitate Ca(OH) 2  CaCO 3  sludge which adsorbs or occludes impurities. The clear liquid, separated from the resultant sludge, is passed to a second precipitation stage where it reacts with a CO 2  and oxygen-containing waste gas at least in part from the aerobic biological treatment stage, thereby forming additional CaCO 3 . Waste gas withdrawn from the second precipitation stage, being enriched in oxygen is recycled to the aerobic biological treatment stage.

BACKGROUND OF THE INVENTION

This invention relates to a process for the purification of wastewaterwherein the wastewater is first introduced into an aerobic biologicaltreatment stage and subsequently into a precipitation stage.

Organically polluted wastewaters are usually subjected to biologicalpurification. After the biological purification, substances which cannotbe biograded, or are biogradable only with difficulties, remain in thewastewater and require removal. This is a particularly important problemin the case of certain industrial effluents, for example cellulosewastewaters. To remove these substances which are refractory tobiological purification, a downstream precipitation stage is employed toocclude and/or adsorb the substances, for example by precipitating withFe salts, Al salts, or bentonite, etc.

Classical methods employing a precipitation stage following a biologicalstage exhibit the drawback that precipitation chemicals are costly andcannot be recovered, thereby adding a significant cost to the system.Moreover, large amounts of organically loaded inorganic precipitationsludge must be dumped, thereby causing secondary expenses and problems.

SUMMARY OF THE INVENTION

An object of one aspect of the present invention is to provide animproved process of the type discussed above.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

According to the process aspect of the invention, a process is providedwhich comprises:

(a) passing the wastewater into an aerobic biological treatment stateand aerating the wastewater with a gas containing at least 21 molarpercent oxygen,

(b) passing resultant biologically treated wastewater in contact withexcess Ca(OH)₂ to precipitate a Ca(OH)₂ +CaCO₃ sludge and to removefurther impurities from the wastewater,

(c) separating resultant Ca(OH)₂ +CaCO₃ from the purified wastewater.

The organically contaminated wastewater is subjected to secondarytreatment, i.e. biological purification in a tank aerated withpreferably technically pure oxygen, at least about 90 molar percentoxygen, or with a gas enriched in oxygen, e.g., at least about 30 molarpercent oxygen. Subsequently, the biologically purified wastewater iscombined with Ca(OH)₂ preferably in a quantity of about 500-6,000 g/m³.

Some of the added Ca(OH)₂ may form insoluble phosphates if phosphatesare present in the wastewater. However, it is essential, according tothe present invention to add substantially greater amounts of Ca(OH)₂than would be conventionally added to remove phosphates. As phosphatecontents of most wastewaters is low (typically below 10 mg/l) the amountof Ca(OH)₂ consumed for Ca₃ (PO₄)₂ precipitation is negligible. If,however, phosphate-concentrations above 100 mg/l are found, anadditional amount of Ca(OH)₂ is added, to overcome the stoichiometricrequirements of Ca₃ (PO₄)₂ -precipitation.

After the Ca(OH)₂ is introduced, a sludge is formed comprisingundissolved Ca(OH)₂ and CaCO₃, the latter being produced by reaction ofCa(OH)₂ with the CO₂ content of the wastewater. An organic pollutantload refractory to secondary treatment is bound to the sludge. Thesludge is then separated, for example, by sedimentation.

The strongly alkaline supernatant is preferably mixed with CO₂-containing gas which can be the waste gas from the aeration facility,the exhaust gas from a lime kiln (calcining furnace), or from ananaerobic reactor, or from a fossil fuel-fired conventional furnace, ora combination thereof. Thereby the wastewater is neutralized and at thesame time additional CaCO₃ is precipitated, binding additional organicpollutant load. The CaCO₃ sludge is likewise separated, for example bysedimentation, whereas the neutralized wastewater is discharged.

The sludges are dewatered and fed into a calcining furnace where theorganic substances, with supplied oxygen, are burned to CO₂, and theCaCO₃ and Ca(OH)₂ are converted into CaO (burnt lime). The latter isslaked with water to form Ca(OH)₂ and reused in the precipitation stage.

The invention achieves a combination of a biological first stage with aphysicochemical second stage in such a way that the specific effects ofboth process stages (high degree of elimination of the organicpollution) remain preserved without introducing secondary problems suchas high costs of chemicals or sludge dumping.

The invention also makes it possible to purify wastewaters having a highproportion of substances which are difficult to biodegrade and/or whichcarry colorants, e.g., dyes, at a high degree of efficiency.

Those wastewater streams that are particularly amenable to the presentinvention analyze as follows:

wastewaters from pulp and papermills (originally no phosphate) where thecolored and refractory compounds are predominantly lignins and ligninderivatives made soluble during the pulping process.

chemical and petrochemical wastes, where all kinds of high molecularweight substances such as polymers are found (no phosphates).

coke oven plant wastes, where phenols, polyphenols, tars prevail (nophosphates).

sugar mill wastes where dextrous and other molasses biopolymers arecolored and refractory.

food industry wastes with Maillard-Products from thermal processes(generally no phosphates).

Since the precipitation sludge, after conversion in the calciningfurnace, can be reused for precipitation, there is no need to dumpprecipitation sludge. Owing to the recirculation of Ca(OH)₂, the needfor chemicals is very low. Furthermore, the oxygen utilized in theaeration facility can be utilized to almost 100%. Since CO₂ isconstantly newly formed by the biodegrading process, and is constantlybeing recovered by the decomposition of CaCO₃, the process isself-sufficient with respect to CO₂.

A wastewater having an especially high degree of purity is produced bythe biological and physicochemical purification process of thisinvention so that it is possible to at least partially recycle manytreated industrial wastewaters. Thereby, not only is the consumption offresh water reduced, but it is likewise possible to lower the amounts ofwastewater passed into drainage ditches. As an indirect result, theplant operator is subject to lower wastewater fees or fines.

BRIEF DESCRIPTION OF DRAWING

The attached FIGURE is a flow chart of a preferred comprehensiveembodiment of the invention, and the following tables of legendsdescribe the various streams and apparatuses set forth:

Streams Identified by Reference Numerals in the FIGURE

1. feed of raw wastewater to the aeration tank B

2. activated sludge/wastewater mixture to the post clarification tank C

3. backflow sludge from the post clarification tank C to the aerationtank B

4. excess biological sludge for discharge (e.g. dewatering andcombustion)

5. biologically pre-purified wastewater from post clarification tank Cto the alkaline mixing tank D

6. alkaline mixture of wastewater and precipitation sludge, undissolvedCa(OH)₂, precipitated CaCO₃, and adsorbed organic material (COD)

7. precipitation sludge Ca(OH)₂, CaCO₃ and organic load from alkalinemixing tank D

8. alkaline clear water from settling tank E

9. neutral clear water and neutral precipitation sludge (CaCO₃ andadsorbed organic material)

10. physically purified wastewater for delivery to the drainage ditch orfor return into production

11. settled neutral precipitation sludge (CaCO₃ and organic material)

12. mixture of precipitation sludges 7 and 11 to dewatering facility H

13. liquid phase of mixed precipitation sludges (centrifugate, filtrate)back into the alkaline mixer D

14. dewatering precipitation sludge to calciner I Ca(OH)₂ and CaCO₃ andorganic material]

15. burnt lime (CaO for reuse

16. slaking water for producing Ca(OH)₂ from CaO, suitably as acomponent stream of the purified wastewater 10

17. Ca(OH)₂ suspension (milk of lime) for recycling into the alkalinemixer D

18. mixture of 17 and liquid phase 13 from dewatering facility forfeeding into the alkaline mixer D

19. CaCO₃ to replenish Ca losses through drainage 10

20. gaseous oxygen for supplying the aerobic biomass in the aerationtank

21. waste gas of the aeration tank, consisting essentially of oxygen andCO₂

22. waste gas from neutral mixer F, consisting essentially of oxygen,for recycling into the aeration tank; CO₂ from the aeration tank reactswith Ca(OH)₂ to insoluble CaCO₃

23. waste gas from the calciner I, essentially CO₂ from CaCO₃ and fromthe combustion of organic material

24. excess CO₂ for removal

Apparatuses Identified by Reference Letters in the FIGURE

A. oxygen supply (e.g. air fractionator, pipeline, liquid oxygen)

B. oxygen-aerated aeration tank

C. post clarification tank for separating activated sludge andwastewater

D. mixing tank for biologically purified wastewater and Ca(OH)₂

E. alkaline settling tank for separating alkaline sludge Ca(OH)₂ andCaCO₃ and adsorbed organic load

F. mixing tank for alkaline wastewater with CO₂ -containing gas

G. neutral settling tank for separating CaCO₃ and adsorbed organic load

H. dewatering device for precipitation sludges from E and G

I. calcining furnace (revolving tubular furnace) for converting Ca(OH)₂and CaCO₃ and organic material into CaO (burnt lime) and CO₂

J. lime slaking tank

K. CaCO₃ reservoir for replenishing lime losses

Detailed Description of the Process of the FIGURE

An organically loaded, approximately neutral wastewater 1 is conductedinto the oxygen-aerated aeration tank B where the biodegradableproportion of the organic load is partially converted into biologicalexcess sludge 4 and partially into CO₂, the latter leaving the aerationtank B together with unconsumed oxygen as waste gas 21. The activatedsludge/wastewater mixture 2 flows into a post clarification tank C whereit is separated into a sludge stream 3 and 4 and into a clear waterstream 5.

The biologically purified wastewater 5 is conducted into an alkalinemixer D where it is mixed with Ca(OH)₂ from a lime slaking tank J. Thebiologically purified wastewater 5 has an increased CO₂ /HCO₃ content,owing to the high CO₂ partial pressure in the gas phase of theoxygen-aerated aeration tank B, leading to the precipitation of CaCO₃ byreaction with excess Ca(OH)₂. The organic material still contained inthe wastewater after the biological purification is extensively bound tothe precipitation sludge due to adsorption on undissolved Ca(OH)₂ andprecipitated CaCO₃ and due to the formation of insoluble Ca salts.

In an alkaline settling tank E, the precipitation sludge 7 is separatedfrom the alkaline clear water 8. The alkaline clear water is conductedinto a neutral mixing tank F and mixed therein with CO₂ -containing gasconsisting essentially of the waste gas 21 from the aeration tank B andpartially of the waste gas 23 from the calciner I. CaCO₃ is produced byreaction of Ca(OH)₂ with CO₂ ; this CaCO₃ is precipitated in insolubleform and adsorbs further organic material. During this step, thewastewater is neutralized, a basic requirement for being discharged to adrainage ditch and/or for recycling, for example, into an industrialfacility. Since CO₂ is extensively reacted to CaCO₃ in the central mixerF, an oxygen-rich waste gas 22, i.e., above the concentration of O₂ inair (21 mol. %), preferably 50 to 99, especially 70 to 95 molar percentO₂, is obtained therein which is returned into the aeration tank,resulting in a practically complete utilization of oxygen, e.g., above95%, preferably above 98% oxygen.

In a neutral settling tank G, the precipitated CaCO₃ with the adsorbedorganic material is separated from the neutral clear water 10. Thesludges from settling tanks E and G are combined and fed to a dewateringfacility H, for example a centrifuge or a filter press. The liquid phase13 is returned into the alkaline mixer where microscopic CaCO₃ particlesact as crystallizing nuclei. The dewatered sludge is heated in acalciner I where it is converted into burnt lime; during this step, theorganic material is simultaneously burned in the presence of oxygen. TheCO₂ -containing waste gas 23 from the calciner I is introduced into theneutral mixer F, insofar as necessary for covering the residual CO₂requirement.

Calcium values discharged with the purified wastewater 10 arereplenished from a reservoir K, suitably in the form of inexpensiveCaCO₃ (limestone) which is fed, together with the precipitation sludge12, to the calciner I. Since CO₂ is constantly formed in the aerationtank B, dissolved in the biologically purified wastewater 5 as well asconcentrated in the waste gas stream 21, and since CO₂ is likewiserecovered from the calcining device, the entire process cycle isself-sufficient with respect to CO₂. Excess CO₂ is suitably transferredout of the calciner waste gas 23.

The disclosures of all applications, patents and publications, if any,cited above and below, and of corresponding German applicationP3917415.8, filed May 29, 1989, are hereby incorporated by reference.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention tois fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

EXAMPLE

Highly colored wastewater from a pulp mill was subjected to an oxygenaerated activated sludge treatment. Average influent COD was around 540mg/l. Biological oxidation only removed some 50%, so that the effluentfrom the secondary clarifier still contained some 270 mg/l of COD andwas still highly colored (dark brown like black coffee). This effluentwas mixed with increasing amounts of Ca(OH)₂. Although small amounts ofCa(OH)₂ as low as 0.3 g/l already showed an effect of COD and colorremoval, optimized results were obtained with 5 g/l of Ca(OH)₂. HigherCa(OH)₂ additions resulted in only marginal improvements. Thus by adding5 g/l of Ca(OH)₂, COD dropped from 270 to 167 mg/l (38% removal), whilecolour (measured as an extinction of 435 nm) was removed by 88%. A brownprecipitate was obtained, which when dewatered and dried, yielded a darkbrown powdered product. The alkaline supernatant (pH 12.2) was testedwith Co₂ -containing gas until a pH of 7.5 was reached. This led to theprecipitation of a whitish precipitate. A further COD removal of 10 mg/land a further extinction decrease of 3% were observed. Afterincineration at 950° C., the combined precipitates yielded a whitepowdered product, which proved to be very pure CaO, which after storingcould be reused as a precipitant, showing the same efficiency for COD-and color-removal as fresh Ca(OH)₂.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdepartment from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A process for the purification of wastewatercomprising:(a) passing the wastewater into an aerobic biologicaltreatment stage and aerating the wastewater with a gas containing atleast 21 molar percent oxygen, to form activated sludge and biologicallypurified wastewater, (b) separating said activated sludge from saidbiologically purified wastewater, (c) combining resultant separatedbiologically purified wastewater with Ca(OH)₂ in a precipitation stageto precipitate a Ca(OH)₂ /CaCO₃ sludge and to remove non-biodegradedimpurities from the wastewater, said Ca(OH)₂ being combined insubstantial excess of stoichiometric quantities for forming Ca₃ (PO₄)₂,and (d) separating resultant Ca(OH)₂ /CaCO₃ from the purifiedwastewater.
 2. A process according to claim 1 wherein a gas containingat least 90 molar percent oxygen is employed to aerate the wastewater inthe biological treatment sludge.
 3. A process according to claim 2,further comprising downstream of the precipitation stage, mixing thepurified wastewater with a CO₂ -containing gas to produce additionalCaCO₃ sludge to adsorb additional impurities, and separating furtherpurified wastewater from said additional CaCO₃ sludge.
 4. A processaccording to claim 1, further comprising downstream of the precipitationstage, mixing the purified wastewater with a CO₂ -containing gas toproduce additional CaCO₃ sludge to remove additional impurities, andseparating further-purified wastewater from said additional CaCO₃sludge.
 5. A process according to claim 4, wherein the CO₂ -containinggas comprises gas from the aerobic biological treatment stage.
 6. Aprocess according to claim 5, further comprising withdrawing from thestage producing additional CaCO₃ sludge, a waste gas containing anoxygen concentration higher than air and recycling said waste gas to theaerobic biological treatment stage.
 7. A process according to claim 6wherein said waste gas contains at 70 molar percent oxygen.
 8. A processaccording to claim 4, further comprising withdrawing from the stageproducing additional CaCO₃ sludge, a waste gas containing an oxygenconcentration higher than air and recycling said waste gas to theaerobic biological treatment stage.
 9. A process according to claim 8,wherein said waste gas contains at least 70 molar percent oxygen.
 10. Aprocess according to claim 9, wherein a gas containing at least 90 molarpercent oxygen is employed to aerate the wastewater in the biologicaltreatment stage.
 11. A process according to claim 10, wherein thenon-biodegraded composition comprises mostly organic matter.
 12. Aprocess according to claim 1, further comprising dewatering said sludgeand calcining resultant dewatered sludge in a calcining furnace wherethe sludge is partially converted into CaO; slaking the CaO with waterto form Ca(OH)₂ ; and reintroducing resultant Ca(OH)₂ into theprecipitation stage.
 13. A process according to claim 1, wherein saidsubstantial excess is 500-6,000 g/m³ of Ca(OH)₂.
 14. A process accordingto claim 13, wherein said wastewater contains above 100 mg/l ofphosphate.
 15. A process according to claim 1, wherein said wastewatercontains no phosphates.
 16. A process according to claim 1, wherein saidwastewater contains less than 10 mg/l of phosphate.
 17. A processaccording to claim 1, wherein the non-biodegraded composition comprisesmostly organic matter.